Very many coating compositions and application methods have already been proposed for achieving the aforementioned protection, these meeting with varied success.
Among all the compositions of the prior art, some owe their properties to the presence of compounds from elements other than the usual constituents of organic matter and, in particular, to aluminum and silicon in the form of specific mineral or organic compounds. With reference to silicon, for instance, some of the techniques used involve the build-up of a protective coating on the substrate, this coating being obtained from the vapor phase deposition of glass or silica evaporated under vacuum. Polysiloxane based protective coatings can also be obtained, the structure of which resembles to some extent that of cross-linked polysilicic acid, by the in-situ polymerization of organo-silicon compounds previously partly hydrolyzed. During the hardening (curing) of such coatings, polymerization occurs, either due to the formation of Si--O--Si bridges (by the dehydration of silanol functions), or due to the participation of polymerizable organic groups belonging to substituents possibly present on the silicon atoms (olefins, epoxy-, amino-groups, etc.), or by a combination of the said two polymerization modes. From the references illustrating such techniques, the following can be cited: A. J. REEDY, Res. Discl. 1978, 171-6; Patents U.S. Pat. Nos. 4,006,271; 4,098,840; 4,186,026; 4,197,335; JP (Kokai) Nos. 77, 101,235; 112,698; 152,426; 154,837; Nos. 79 60,335; 62,267; 119,597; 119,599; 129,095 to 129,099; 133,600; 144,500; 148,100; Nos. 80, 05,924; and DOS Nos. 2,803,942; 2,805,552; 2,820,391; 2,831,220; 2,917,440.
Despite the protection they impart to the substrate on which they are applied, these coatings have drawbacks. One of such drawbacks or disadvantages is related to the relatively high temperatures needed for curing polysilicic type coatings which can lead to substrate deformation. Another drawback is related to the expansion coefficient of the polysiloxane coatings which is often sufficiently different from that of the substrate to cause the development of adhesion problems (for instance in the case of polycarbonate or polymethacrylate organic glasses) and of cracks or crazing after alternating hot and cold periods (particularly in the case of articles subjected to weathering like automobile headlights). Adhesion problems of this type were attempted to be solved by interposing an intermediate bonding sublayer between the coating and the substrate. More generally the above mentioned drawbacks have been attempted to be remedied by replacing the coatings of polymerized silicon compounds with compositions comprising, dispersed within an organic or silicon-organic matrix, fine particles of silica or alumina, specifically, the compositions relied upon have been aqueous mixtures of silicon compounds, colloidal silica and hydrocompatible solvents (alcohols, glycols, etc.), with or without polymerizable organic monomers. Examples of such uses can be found in the following references: Belgian Pat. Nos. 821,403; 877,372; U.S. Pat. Nos. 4,027,073; 4,188,451; 4,177,315; GB Nos. 2,018,621; 2,018,622; DOS No. 2,811,072 and JP (Kokai) No. 79, 157,187.
Colloidal silicas which are essentially hydrophilic (as are also the other types of silica such as amorphous, crystalline, microcrystalline, precipitated and pyrogenic silicas) are compatible in general, only with hydrophilic polymers. Such silicas are much less or not miscible with typical hydrophobic resins such as polyolefins, which very strongly restricts their use as a filler in film forming thermosetting or photo-setting compositions. Moreover, adding hydrophilic silica to organic polymerizable monomers leads to the formation, with relatively low concentrations of solids, e.g. about 5 to 10% by weight, of highly thixotropic masses (non-Newtonian rheologic behavior) which are very difficult to apply as thin layers on substrates. Hence, attempts were made to remedy this disadvantage, i.e. to increase the level of silica in organic resin coatings, while overcoming such application problems, by treating the particles so as to make them organophilic. One of the most promising methods for achieving such purposes, i.e. for imparting hydrophobic organophilic properties to alumina or silica particles sufficient to enable them to be incorporated at high levels (of about 10 to 40% by weight or more) into polymeric resin films, while maintaining suitable rheological properties for application and translucency or transparency of the films formed, consists in attaching suitable organo-compatible substituents to said particles. (The terms compatible and compatibility as used herein shall be understood to mean that the filler must be made compatible with the hydrophobic vehicle, i.e., it must be made oleophilic, highly dispersible without aggregation into the vehicle to form a finely divided dispersion either transparent or translucent depending on the degree of matching of refractive index, and capable of addition at high concentration without forming viscous or thixotropic solutions). The substituents must be selected so as to ensure proper transparency of the protective coatings without detracting from resistance to abrasion. As pertinent references regarding the methods for "treating" silica or alumina particles for rendering them organophilic, South African Pat. No. 72.5180 and Japanese Patent (Kokai) No. 77, 138,154 can be cited. In the first of these references, silica particles are treated with trimethylchlorosilane which, by reaction with the silanol groups of said particles, generates hydrophobic groups of formual ##STR1## whereby said particles are rendered compatible with a mixture of olefinic monomers (ethylenic and acrylic monomers). These particles are then incorporated, to a level of about 5-10% by weight and together with a proportion of alumina about 10 to 20 times greater, into a mixture of polymerizable resins which, after curing, provides insulators for high electric voltages. Such materials are however opaque and their resistance to abrasion is not indicated. In the second of the two references cited above, particles of alumina are coated with .gamma.-(glycidyloxy)-propyl-trimethoxysilane and a mixture containing about 25% by weight of such treated alumina and an epoxy resin is used for coating a polycarbonate article so as to obtain, after polymerization, an abrasion-resistant film. Also, in the following references, there are described methods for attaching organic groups such as vinyl, methacryl, epoxy, glycidoxy to hydrophilic silica so as to impart thereto hydrophobic properties: L. P. ZIEMIANSKI et al, Rubber World 163, 1 (1970); M. W. RANEY et al, Meeting of the Div. of Rubber Chem., ACS Meeting, Cleveland, Ohio (1971); M. W. RANEY et al, Meeting of the Div. of Rubber Chem., ACS, Miami, Fla (1971); HI-SIL Bulletin 41, Jan. 1971, PPG Industries.
In addition to the above mentioned prior art, some further U.S. Patent references can be cited in connection with the following subjects related more or less closely to the invention:
1. SiO.sub.2 : U.S. Pat. Nos. 3,986,997; 4,177,315; 4,188,451; 4,242,403. PA0 1A. Treated SiO.sub.2, e.g. to make it hydrophobic: U.S. Pat. Nos. 2,610,167; 2,818,385; 3,652,379; 4,001,128. PA0 2. Forming SiO.sub.2 in situ, e.g. hydrolyzing organic silicates: U.S. Pat. Nos. 2,404,357; 2,404,426; 3,971,872; 4,049,868; 4,120,992; 4,186,026. PA0 3. Using siloxanes and/or silanes and the like: U.S. Pat. Nos. 2,610,167; 3,389,114; 3,801,361; 3,953,115; 3,986,997; 4,001,128; 4,006,271; 4,026,826; 4,027,073; 4,029,842; 4,049,868; 4,177,315; 4,186,026; 4,188,451; 4,197,335; 4,242,403. PA0 4. Combination of any of the above items with: PA0 4A. Polymers: U.S. Pat. Nos. 2,404,357; 2,404,426; 2,610,167; 3,324,074; 3,328,339; 3,419,517; 3,652,379; 3,801,361; 3,971,872; 4,001,128; 4,026,826; 4,049,868; 4,098,840; 4,120,992; 4,197,335; 4,242,403. PA0 4B. Prepolymers (oligomers or monomers) suitable as organic phases for abrasion-resistant films: U.S. Pat. Nos. 3,819,562; 4,029,842; 4,197,335. PA0 4B1. Photopolymerizable monomers: U.S. Pat. Nos. 3,968,305; 3,968,309; 4,188,451 PA0 4C. Other chemicals, e.g. solvents, fillers, cross-linking agents, to obtain transparent abrasion-resistant coatings (as single or composite systems): U.S. Pat. Nos. 3,986,997 (acidic alcohol H.sub.2 O solution); 4,001,128 (Al.sub.2 O.sub.3); 4,006,271 (solvent); 4,027,073 (acidic alcohol water solution); 4,049,868; 4,186,026 and 4,120,992 (cross-links with formaldehyde); 4,120,992. PA0 5. Miscellaneous routes to such coatings: thus U.S. Pat. No. 3,645,779 provides a vacuum vapor deposited coating of B.sub.2 O.sub.3 -SiO.sub.2 on organic glass; U.S. Pat. No. 4,051,297 discloses a sputtered film of chromium silicide on smooth surfaces; in U.S. Pat. No. 4,242,403, there is disclosed a polyethylene terephthalate sheet covered with an intermediate layer of .gamma.-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane and an upper layer of silica reinforced organopolysiloxane resin. PA0 depositing of transparent protective films on substrates, such films being sufficiently mechanically resistant to withstand normal wear or accidental abuses without impairment of the surface properties; providing protective transparent film coatings on optical goods, the optical properties of which are not significantly modified and which keep such properties for a significant period of time under adverse conditions; depositing of thin well-adhering films on substrates, such adhesion not being affected by weathering conditions even after a prolonged period of exposure; providing film forming coatings that will strongly adhere to organic glass substrates and which can be cured at room temperature, i.e. much below the softening temperatures of the substrate; providing transparent scratch-resistant coatings applied on substrates as one layer films, i.e. without the need of an intermediate bonding layer; providing compositions stable enough for accepting prolonged storage periods at room temperature without hardening and nonetheless reactive enough to be cured on the substrates in a matter of seconds witnhout the use of elevated temperatures. PA0 in which R represents identical or different lower alkyls and R.sup.1 and R.sup.2 are identical or different organic substituents compatible with said organic phase, n being 0 or 1, said hydrolysis providing a corresponding di- or trihydroxy-silane of formula EQU (HO).sub.3-n SiR.sup.1 R.sub.n.sup.2 (II) PA0 in solution in the hydrolysis medium, PA0 vinyltriethoxysilane PA0 vinyl-tris(beta-methoxyethoxy)silane PA0 gamma-methacryloxypropyltrimethoxysilane PA0 beta-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane PA0 gamma-glycidoxypropyl-trimethoxysilane PA0 gamma-aminopropyltriethoxysilane PA0 N-beta-(aminoethyl)-gamma-aminopropyl-trimethoxysilane PA0 gamma-ureidopropyl-triethoxysilane PA0 gamma-chloropropyltrimethoxysilane PA0 gamma-mercaptopropyl-trimethoxysilane PA0 N-.beta.-(N-vinylbenzyl-amino)ethyl-.gamma.-aminopropyltrimethoxy-silane monohydrochloride PA0 vinyltrimethoxysilane CH.sub.2 :CHSi(OCH.sub.3).sub.3 PA0 vinyltriethoxysilane CH.sub.2 :CHSi(OC.sub.2 H.sub.5).sub.3 PA0 vinyl-tris-(.beta.-methoxy-ethoxy)-silane CH.sub.2 :CHSi(OC.sub.2 H.sub.4 OCH.sub.3).sub.3
In addition to the aforementioned prior art, in copending International Application No. PCT/EP82/00004, a composition was disclosed for providing thin translucent or transparent films very resistant to abrasion by virtue of a high level therein of hydrophobic silica. This composition comprises an organic phase consisting of one or more photopolymerizable monomers or prepolymers, at least one photo-initiator and charges of SiO.sub.2 or Al.sub.2 O.sub.3 particles having, grafted on some of the oxygen atoms thereof, silane or siloxane substituents. By means of said composition the following desirable objectives were achieved:
The fulfillment of these objects by the reference disclosure provided industrial optical articles made of relatively soft and easy moldable organic glasses protected with a scratch resistant film withstanding prolonged use under severe weathering conditions without discoloration, crazing or significant adhesion losses.
In one of the preferred methods of manufacturing the composition disclosed in PCT/EP82/00004 the mineral particles to be incorporated into the organic phase are rendered hydrophobic by reacting with trialkoxysilane compounds. In this method, the trialkoxysilane compounds are first dispersed into an aqueous medium (mainly a water phase) wherein they are converted to some extent by hydrolysis into corresponding hydroxysilane compounds. Upon subsequent addition of the mineral particles (SiO.sub.2 or Al.sub.2 O.sub.3) a condensation reaction between the hydroxy groups of the hydroxysilanes and that of the particles occurs with consequent attachment of said silane compounds onto the particles. This is essentially the generally accepted description of the grafting phenomenon involved in this reaction which leads to the production of hydrophobic graft mineral particles.
Then, the grafted charge must be separated from the aqueous reaction medium and dried to an extent sufficient to enable it to be subsequently incorporated into the organic phase of the composition while avoiding difficulties inherent to the presence of an excess of moisture. In the above referred work, this aim was achieved by centrifuging the aqueous dispersion of grafted particles in order to separate the heavier solids from the liquid, then by drying the separated solids in an oven and finally by grinding the dried solids in a mill to the desired mesh size, this last operation being intended to break the agglomerates of particles formed during the drying stage.
It was however found later that this separation and drying step has shortcomings. For instance, during centrifuging, all liquids including possible still ungrafted hydroxysiloxane are eliminated. This elimination is undesirable because of loss of reagent and overall decrease of the grafting yields. Also, during the drying stage in an oven, the heat developed there can possibly damage, at least in part, the organic substituents grafted on the particles or, in the case when such substituents carry reactive functions to be reacted later with monomers in the organic phase upon curing, such functions may prematurely react together and produce unwanted early cross-links between the substituents. These conditions change the rheological properties of the composition upon mixing of the grafted particles with the organic phase (high viscosity build-up at relatively low levels of mineral charge incorporation). Finally, the drying stage in an oven appears to be responsible for the formation of relatively large particle aggregates requiring the dried product to be ground for long periods before incorporation into the organic phase, a step which is economically undesirable. (The term "charge" as used throughout this application is used in the sense referring to material load or weight.)
The first drawback mentioned was cured by replacing the centrifuging step by an evaporative step (for instance in a conventional Rotavapor apparatus) of the aqueous grafting medium (under ordinary or reduced pressure), but this did not remove the agglomeration problems nor the problems associated with premature reaction since evaporation of large amounts of water requires prolonged heating periods even under relatively low pressures.